I have a battery powered lawnmower. It’s powered by a 17Ah 24V battery which lasted four seasons before it just wouldn’t finish the lawn. The replacement battery started giving me trouble by the end of the first season. It was barely adequate and had developed a strange quirk where it would spin the motor up slowly the first time it was powered up. The brushes seemed to have plenty of wear left, so as an experiment I tried touching the motor wire directly to the battery terminal. It spun up instantly, like it was new. This made me suspect the switch contacts, which turned out to be pretty chewed up. At this point, like any normal person would, I decided to have the machine serviced by an authorized service center. Just kidding. This mower literally consists of a motor, a switch, and a battery. I decided to replace the switch assembly with a MOSFET.

A simple lawn mower control circuit

Based on the observed battery life and the 40A circuit breaker in the original circuit, I estimated around 20A of current draw.

The IRF1405 is an N-Channel MOSFET intended for automotive applications. It runs $2.45 from Jameco in single quantity. It claims a maximum continuous current of 169A, however the fine print in the datasheet indicates that the TO-220 package limits it to 75A. I’ll be well within that limit, so no problem there. It needs 10V on the gate to turn fully on, while the max allowed is 20V. In the schematic, when the switch is on, the zener diode provides a nice consistent 12V to the gate of Q1. With the switch off, the gate is discharged through R3. I’m not too concerned about the switching speed in this application. I added a small gate resistor, R2, though it’s probably not necessary here. R4 is a 50A, 75mV current shunt that I will use in a future project to add some instrumentation. Since the motor is an inductive load, flyback diode D4 is required to protect the FET. I used two MBR745 diodes in parallel here because I wasn’t sure how to size a motor flyback diode. This turned out to be overkill.

I mounted the entire circuit to a piece of plywood, and the entire unit was temporarily mounted to the top of the battery with a piece of double sided tape:

The finished project shown charging

The board to the right of the battery is the mower’s original charger, balanced precariously and attached with tiny alligator clips. The small wire going out of the picture to the upper left goes to the on/off switch. The two flyback diodes are mounted to a heat sink (scavenged from an old video card) at the top of the main board. Below that is a terminal strip, then the MOSFET also on a heat sink. To the left of the FET is the current shunt. I’ll probably add a 40A breaker when I permanently mount the board.

Yesterday was its inaugural mow. I’m happy to report that it worked perfectly. It mowed the whole yard no problem, and neither the diodes nor the FET even got warm to the touch. The voltage drop across the shunt read 16mV on a fresh battery and no load. This works out to 24A, pretty close to what I was expecting. The longevity of this circuit remains to be seen, but so far I would call it a success. Plus, I’m the only one in the neighborhood with wires and plywood attached to his lawnmower.